This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Membrane fusion is the essence of cell growth (in both normal cells and cancerous cells), hormone secretion, and neurotransmission. Despite its fundamental character, the mechanisms of how membranes fuse are only slowly becoming understood. We study this process with vacuoles (lysosomes) from baker's yeast for the unparalleled power of experimental tools available: vacuoles are large and can be visualized in the living cell, their fusion can be essential or dispensable for viability (depending on the cell growth conditions), the genetics of yeast is well-advanced, we can assay vacuole fusion in vitro with quantitative, colorimetric assays, and the fusion reaction has been reconstituted with all-purified components. The mechanisms we discover, for the Rab-family GTPase, for its effector complex, and for SNAREs, are of general importance for understanding human physiology and pathophysiology, for these same proteins are essential for membrane fusion in humans.